[llvm-dev] Using C++14 code in LLVM

Zachary Turner via llvm-dev llvm-dev at lists.llvm.org
Sat Oct 1 22:33:40 PDT 2016

A thread was started over the summer to discuss the timeline for bumping
LLVM up to Visual Studio 2015 to enable the use of various new language
features.  Currently the ETA for this is sometime in mid-October, so within
2-3 weeks.

With this happening imminently, I thought it would be worth tossing this
out there and seeing what people think.

With VS on 2015, the major lagging compiler is going to be GCC.  Our
minimum GCC requirement is 4.7, which is quite old (almost 4 years for
anyone keeping count).  What are the challenges with pushing this forward?

With VS2015, the biggest remaining missing C++14 features are variable
templates, sized allocation, and extended constexpr.  But it gains generic
lambdas, init-captures, decltype(auto) return, and auto return without
requiring the trailing -> decltype syntax.

While GCC doesn't claim to "fully" support C++14 until 5.2 (which is only
about 1 year old), you can get all of the above features with GCC 4.9

So if we're willing to allow "partial" C++14 support in LLVM (i.e a
whitelisted set of features), we may be able to get many of the most useful
features by going to GCC 4.9, which is still a good 2.5 years old.

One potentially added benefit of this is that GCC supports <regex> in 4.9.
This might allow us to kill of llvm::Regex in favor of standardizing on
std::regex, as GCC is currently the only supported compiler without a regex

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